Electrical Hazards

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Electrical Hazards. Electrical Hazards. What are the hazards as you approach electrical equipment to perform work?. Electrical Hazards. Shock Arc flash Heat Fire Arc blast Pressure Shrapnel Sound. Example of an arcing fault. Basic Electrical Theory. I = V / Z

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Electrical Hazards

Electrical Hazards

What are the hazards as you approach

electrical equipment to perform work?

Electrical Hazards
• Shock
• Arc flash
• Heat
• Fire
• Arc blast
• Pressure
• Shrapnel
• Sound

Example of an arcing fault

Basic Electrical Theory

I = V / Z

• What happens with shock?
• What happens when there is a fault?
• What is the difference between a short-circuit and an arcing fault?
Electric Shock
• Over 30,000 non-fatal electrical shock accidents occur each year
• Over 600 people die from electrocution each year
• Electrocution remains the fourth (4th) highest cause of industrial fatalities
• Most injuries and deaths could be avoided
Electric Shock

Human body resistance (hand to hand) across the body is about 1000 W

Ohms law: I = V / R amps

= 480 volts / 1000 W

= 0.48 amps (480 mA)

Product safety standardsconsider 5 mA to be the safe upper limit for children and adults

Electric Shock

mAAffect on person

0.5 - 3 - Tingling sensations

3 - 10 - Muscle contractions and pain

10 - 40 - “Let-go” threshold

30 - 75 - Respiratory paralysis

100 - 200 - Ventricular fibrillation

200 - 500 - Heart clamps tight

1500 + - Tissue and organs start to burn

Current passing through the heart and lungs is the most serious

Electric Current Pathways
Arc Flash

As much as 80% of all electrical injuries are burns resulting from an arc-flash and ignition of flammable clothing

Arc temperature can reach 35,000°F - this is four times hotter than the surface of the sun

Fatal burns can occur at distances over 10 ft

Over 2000 people are admitted into burn centers each year with severe electrical burns

Arc Blast

An arc fault develops a “pressure wave”

Sources of this blast include:

• Copper expands 67,000 times its original volume when vaporized
• Heat from the arc, causes air to expand, in the same way that thunder is created from a lightning strike

This may result in a violent explosion of circuit components and thrown shrapnel

The blast can destroy structures, knock workers from ladders, or across the room

A

B

A

B

BoltedShort Circuit

Arcing

Fault

Current Thru Air

Electric Arc

Molten Metal

35,000 °F

Pressure Waves

Sound Waves

Shrapnel

Copper Vapor:

Solid to Vapor

Expands by

67,000 times

Hot Air-Rapid Expansion

Intense Light

Personnel Hazards Associated With Arc Flash & Arc Blast
• Heat – burns & ignition of material
• Arc temperature of 35,000oF
• Molten metal, copper vapor, heated air
• Second degree burn threshold:
• 80oC / 175oF (0.1 sec), 2nd degree burn
• Third degree burn threshold:
• 96oC / 205oF (0.1 sec), 3rd degree burn
• Intense light
• Eye damage, cataracts
Personnel Hazards Associated With Arc Flash & Arc Blast
• Pressures from expansion of metals & air
• Eardrum rupture threshold:
• 720 lbs/ft2
• Lung damage threshold:
• 1728 - 2160 lbs/ft2
• Shrapnel
• Flung across room or from ladder/bucket
Overcurrent Protection Role

Flash protection boundaries and incident energy exposure calculations both dependent upon:

Duration of arc-fault or time to clear

• Speed of the overcurrent protective

device

Arc-fault current magnitude

• Available fault current
• Current-limitation can reduce
IEEE / PCIC & NFPA 70E
• Users
• Consultants
• Manufacturers
• Medical experts
• Following are some of the tests run
• All of the devices used for this testing were applied according to their listed ratings

22.6 KA Symmetrical

Available Fault Current

@ 480V, 3 Phase

Test No. 4

6 cycle STD

640A OCPD

Non Current Limiting

with Short Time Delay

Set @ 6 cycle opening

Fault Initiated on

Line Side of 30A

Fuse

30A RK-1

Current Limiting Fuse

Size 1 Starter

Results: Test No.4

Sound

141.5 db @ 2 ft.

P1

T2

>2160 lbs/ft2

>225oC/437oF

T1

>225oC/

437oF

T3

50oC/122oF

> Indicates Meter Pegged

22.6 KA Symmetrical

Available Fault Current

@ 480V, 3 Phase

Test No. 3

601A.

Class L

Current Limiting Fuse

Fault Initiated on

Line Side of 30A

Fuse

30A RK-1

Current Limiting Fuse

Size 1 Starter

Results: Test No.3

Sound

133 db @ 2 ft.

P1

504 lbs/ft2

T2

62oC/143.6oF

T1

> 175oC/

347oF

T3

(No Change

From Ambient)

> Indicates Meter Pegged

22.6 KA Symmetrical

Available Fault Current

@ 480V, 3 Phase

Test No. 1

601A.

Class L

Current Limiting

Fuse

30A RK-1

Current Limiting Fuse

Fault Initiated on

Fuse

Size 1 Starter

Results: Test No.1

Sound

(No Change From Ambient)

P1

(No Change From Ambient)

T2

(No Change From Ambient)

T1

T3

(No Change From Ambient)

(No Change From Ambient)

Current-Limitation: Arc Energy Reduction

Test 4

Non-Current Limiting

Test 3

Reduced Fault Current

via Current-Limitation

Test 1

Summary
• Shock, arc flash and arc blast are the three recognized electrical hazards
• Shock injuries result from electrical current flowing through the body
• Arcing faults can generate enormous amounts of energy
• Injuries from arcing faults are a result of the tremendous heat and pressure generated
Summary
• Overcurrent protective devices have an impact on the two most important variables of arc flash hazards:
• Time (speed of the OCPD)
• Fault current magnitude (current-limitation may help reduce)
• Current-limitation may be able to significantly reduce the energy released during arcing faults